A computation fluid dynamic (CFD) simulation of a rectangular clarifier is performed in this study using a drift-flux model in OpenFOAM CFD code. Using this model with turbulence model, the key characteristics (re-circulation and sedimentation) of water-particle mixture flow in a rectangular clarifier are reasonably reproduced. A fairly good agreement is obtained between the simulation results and experimental data of the velocity profiles. Thus, with the demonstrated capability of this CFD model for the prediction of hydrodynamic and sedimentation behavior of water-particle mixture flow, several design issues such as the determination of the best location of baffle in a clarifier can be investigated and addressed. This method can not only provide general conceptual information at the initial design stage but can also be used to perform analysis of different configurations and the effect of changes in operational parameters.